It has become common practice to manufacture food patties at a central location for distribution to restaurants, fast-food establishments, and other retail outlets including grocery stores. The most prevalent food patties, by far, are hamburger patties molded from gound meat. Other food products are also processed by the same techniques, however, including fish patties, patties formed from flaked or shredded meat, and even patties formed from vegetable foods. The term "food product", as used throughout this specification and the appended claims, is intended to refer to any of the various foods identified above and to any others having similar properties; the food products processed under the invention are not free-flowing, but are quite viscous and resistant to flow, and are only moderately compressible.
In molding food patties, a supply of the food product from which the patties are to be formed is usually maintained in a hopper or similar container. From the hopper the food product is fed to a food pump. Most applications employ a positive feed mechanism that effectively forces the food product from the supply into the pump intake, due to the viscous, flow resistant propeties of the food product. The pump forces the food product, under pressure, into a mold cavity in a mold plate. The mold plate is moved through a cyclic motion, rotary or reciprocating, between a fill position at which it receives food product from the pump and a discharge position at which the food patty is discharged from the mold cavity. In most instances, a 1:1 ratio is maintained between the cyclic operations of the mold plate and the pump. On the other hand, much higher ratios have been used, employing pumps with a capacity substantially greater than that of the mold cavities filled in each pump cycle; see U.S. Pat. No. 3,887,964.
One of the problems presented in conjunction with conventional food patty molding techniques, as briefly discussed above, results from "tumbling" or "churning" of the food product by the feed mechanism that supplies the food product to the pump. This results from the positive feeding action used to force the food product into the pump intake, and the attendant difficulty in avoiding circulatory flow because the feed mechanism cannot be precisely matched to the intake capacity of the pump. This "churning" may cause separation of the food product (e.g., separation of fat from other tissue in ground meat, or separation of water from a fish food product). In addition, the churning effect of the feeder mechanism may grind the food product into smaller particles than desired. In both instances, an undesired deterioration of the food product may result.
Another common problem presented in conventional food patty molding operations is the formation of undesirable bulges or "lips" on the food patties. It is customary to force the food product into the mold cavity under substantial pressure. Air entrained in the food product tends to expand when the food patty is first exposed to atmospheric pressure. This expansion occurs unevenly, concentrated in the portion of the patty that is first exposed. In some food patty molding machines, special post-molding expedients have been adopted to minimize the formation of these bulges on the patties (see U.S. Pat. No. 3,479,687), introducing additional complexity and expense in the manufacture and maintenance of the food patty molding equipment. Furthermore, these bulge-elimination expedients have not been uniformly successful.
Because the food products to which the present invention is directed exhibit poor flow characteristics, it has usually been necessary to utilize high pressures in patty molding equipment affording high volume production; thus, in commercial embodiments of the patty molding machine of U.S. Pat. No. 3,887,964, pressures as high as 400 pounds per square inch may be employed. The pressure requirements are particularly high whenever any valving is interposed between the food pump and the mold cavities. These high pressure levels may produce some deterioration of the food product, of the kind discussed above with respect to churning by the pump feed apparatus, and may also add materially to the overall cost of the molding equipment.